Many mobile machines include a lifting boom that may be swung up and down by means of a double acting hydraulic lift cylinder that acts between the lifting boom and frame work or base of the machine. This particular lift cylinder is included in a hydraulic system comprising a hydraulic pump and a hand valve, by means of which the pump may be connected to the first lift cylinder chamber when the boom is to be elevated and to the second lift cylinder chamber when the boom is to be sunk. Simultaneously, in the first case the second lift cylinder chamber, and in the second case the first lift cylinder chamber is, via the hand valve, connected to a tank for the hydraulic fluid.
Thus, in the most basic embodiment, the hydraulic valve device is such arranged that the pump fills the first lift cylinder chamber when the boom is to be elevated or sunk, such that the hydraulic fluid that is pressed out from the other lift cylinder chamber is released to the tank. Depending on if the boom is moved with or against the load, the pump will have to work much or less in order to achieve the necessary pressure for the operation. However, it must always deliver a sufficient flow to fill the emptying lift cylinder chamber in a pace that allows movement of the boom in the speed desired by the operator.
An unsatisfactory problem of an arrangement of the described type is that it makes the efficiency of the hydraulic system low at lowering of a load since the pump delivers pressure and flow even though the boom could be sunk by means of its own weight and load.